It is often desirable to know the location of a terminal such as a cellular phone. For example, a location services (LCS) client may desire to know the location of a terminal in the case of an emergency services call or to provide some service to the user of the terminal such as navigation assistance or direction finding. The terms “location” and “position” are synonymous and are used interchangeably herein.
In Observed Time Difference of Arrival (OTDOA) based positioning, the mobile station may measure time differences in received signals from a plurality of base stations. Because positions of the base stations are known, the observed time differences may be used to calculate the location of the terminal To further help location determination, Positioning Reference Signals (PRS) are often provided by a base station (BS) in order to improve OTDOA positioning performance. The measured time difference of arrival of the PRS from a reference cell (e.g. the serving cell) and one or more neighboring cells is known as the Reference Signal Time Difference (RSTD). Using the RSTD measurements, the absolute or relative transmission timing of each cell, and the known position(s) of BS physical transmitting antennas for the reference and neighboring cells, the Mobile Stations′ (MS′) position may be calculated.
To facilitate OTDOA based positioning, a location server may include assistance data for at least one cell, for which cell timing (System Frame Number (SFN)) may be obtained by the Mobile Station (MS). The assistance data may pertain to an assistance data reference cell or one or more cells selected from a neighboring cell list. Typically, OTDOA assistance data pertaining to the serving cell is provided because cell timing (such as the System Frame Number) information for the serving cell is generally available to the MS.
However, in some situations, the serving cell cannot be included in the OTDOA assistance data. For example, the serving cell for the MS may be a Femto cell, whose location and timing may not be known to the location server. In other instances, PRS may not be configured for Femto cells. As a further example, the MS serving cell may belong to a frequency layer operating at frequency f1, while PRS′ or assistance data cells are deployed on inter-frequency layer operating at frequency f2.
Therefore, there is a need for apparatus, systems and methods to improve position determinations and permit the use of PRS signals for location determination in situations where the serving cell cannot be included in the OTDOA assistance data.